Mustard Algae: Why the Yellow Patch Keeps Coming Back

It is not sand, and brushing is the test

Mustard algae looks like dirt. It sits in flat, dull yellow or yellow brown patches, it favours the shaded wall and the deep end corners rather than the sunny side, and it comes away from the surface with almost no resistance. That last part is what fools people, because the wall looks clean immediately and the reading on the test kit looks fine.

Brush a patch and watch it. Algae lifts into the water as a cloud that drifts. Sand and pollen swirl and settle straight back down where they were. If it clouds, drifts, and is back on the same wall by the next afternoon, you are looking at algae rather than debris.

Why it comes back, and why it is not your dose

Two things make this different from ordinary green algae, and neither of them is arithmetic.

The first is that it lives out of the water. Damp swimsuits, foam floats, the brush head, the leaf net, the pole, the vacuum hose: anything porous that went into the pool can carry it, and anything that carries it puts it back the next time it goes in. A pool treated perfectly and then stirred with an untreated brush is a pool that gets reinfected on the same day.

The second is that it tolerates chlorine better than green algae does. A routine weekly dose knocks the visible patch off the wall without finishing the population that is left in the corners, which is why the surface looks solved and the pool is not.

So the sequence matters more than the chemical. Decontaminate everything that touched the water, brush the whole surface, then chlorinate, then leave the filter running.

What to add, and how much

Our pool chlorine calculator works from the quantity each product needs to raise free chlorine by 1 ppm in 10,000 gallons, then scales that to the volume you enter. For a 15,000 gallon pool it comes out at:

Run that pool from 1 ppm up to 4 ppm, which is the calculator's own default, and it asks for 45.9 fl oz of the 12.5% liquid, a little over a third of a gallon. Each additional ppm on top is another 15.3 fl oz of the same product. A higher target is not a different method, it is that figure multiplied.

The target itself is the number this page is not going to hand you, because it is not ours to give. It depends on your stabilizer reading, since cyanuric acid holds chlorine back as well as protecting it from sunlight, and on what the label of the product in your hand says to do about algae. If you have never tested stabilizer, read what cyanuric acid actually does first. What the calculator gives you is the cost of whatever target you settle on, in the units printed on the bottle.

All of that assumes you know your volume. If yours is a guess, the pool volume calculator works it out from your dimensions, and it is worth doing once: an estimate that is off by a fifth makes every dose you ever add off by a fifth with it. Product strength matters too, since chlorine loses potency as it ages on a shelf, so an old jug delivers less than its label claims.

Brushing and filtering are not the optional part

Brush the whole pool with a stiff brush, not just the patches you can see, and pay attention to the shaded wall, the corners, the steps and the area behind the ladder. Brushing is what lifts the algae into the water where the chlorine can reach it, and skipping it is why a large dose can still fail.

Then leave the pump running continuously rather than on its usual schedule until the water clears. The filter is what actually removes the dead algae, and water that goes cloudy afterwards is the treatment working rather than failing. If it stays that way, our guide to clearing cloudy pool water is about filtration and circulation rather than chemistry, which is where the answer normally is.

Decontaminate, or expect it back

Do this in the same session as the treatment, not afterwards.

• Swimsuits and towels, washed hot rather than rinsed and hung up.
• Floats, toys and anything foam, which hold water inside them and are easy to forget.
• Brush, net, pole and vacuum hose, including the head, which is where it hides.
• The filter itself, cleaned or backwashed once the water has cleared, because everything you removed from the walls is now sitting in it.

A pool cleared of mustard algae stays clear if nothing carries it back in. That is the whole job, and it is why the equipment step is not housekeeping around the edges of the real treatment. It is half of the real treatment.